Integrating circuits



Aug. 16, 1955 E. w. PULSFORD INTEGRATING CIRCUITS Filed Aug. 16, 1950 0)3 9 v R Q Wm flu a m m 2 2 w NM W mW W np 1 m. $5 A W .5 m A A {E my 11B mmu W W k A 2 n 3 2 United States Patent INTEGRATING cmcurrs EdgarWilliam Pulsford, Strand, London, England, as-

signor, by mesne assignments, to National Research DevelopmentCorporation, London, England, a corporation of Great Britain ApplicationAugust 16, 1950, Serial No. 179,766

3 Claims. 01. s24 7s This invention relates to pulse rate measuringcircuits.

The pulse rate measuring circuit of the invention comprises a condenserconnected to be alternately charged through one valve and dischargedthrough another valve into a reservoir smoothing condenser provided witha leakage path at an incidence controlled by the pulses the rate ofwhich is to be measured, this condenser and leak being connected in theinput circuit of an amplifier having negative feedback to make the inputimpedance comprising the smoothing circuit approach zero. Whenequilibrium is established the mean smoothed direct current in theleakage path is equal to the mean current of the discharges of the firstmentioned condenser, which may be termed the feed condenser, and thevoltage difference between the ends of the leakage path provides ameasurement of the mean current of the discharges of the feed condenserand hence of the number of discharges per unit of time; the measurementof the voltage difierence between the ends of the leakage path isperformed by the negative feedback amplifier in which it is connected:the reservoir smoothing condenser and its leakage path, in addition toperforming a smoothing function on the input current pulses, determinethe interval over which the average of the current is found.

The circuit of the invention may be adapted to measure difierent rangesof input pulse rates or to integrate over any of a series of intervalsby switching means arranged to change the etfective values of thecondensers. According to a subsidiary feature of the invention,condensers to be selectively switched into the circuit as smoothingcondenser are maintained charged to the same voltage, that is to say,the stand-by condensers are kept at the voltage of the effectivesmoothing condenser, so that sudden voltage changes in switching aresubstantially avoided.

The circuit is effective to sum, over a chosen period, voltage pulses orvoltage steps applied to charge the feed condenser.

It is advantageous, especially for pulses of high repetition frequency,to apply the initial pulses to a divideby-two circuit to develop a pulsetrain of which the leading edges (or steps of one sense) coincide withone set of alternate initial pulses and the trailing edges (or steps ofopposite sense) coincide with the other set of alternate initial pulses.

Accordingly, in another aspect the invention resides in a pulse ratemeasuring circuit comprising a constant amplitude square wave generatingcircuit having two stable states connected to be excited by an initialpulse train to develop a sharply stepped wave, the successive steps ofwhich coincide with successive pulses of the initial pulse train, and aswitch circuit having a condenser connected to be charged through onevalve by the voltage steps of like sense, and to be discharged into asmoothing condenser through another valve, the smoothing condenserhaving a leakage path the current in which 2,715,712 Patented Aug. 16,1955 is measured by measurement of the voltage difierence between theends of it.

A rate meter embodying the invention will now be described withreference to the single figure of the accompanying drawing which is aschematic diagram of my invention.

The particular embodiment about to be described is designed to countpositive going pulses of between 5 and 10 volts amplitude applied to aninput terminal IN. These input pulses are shaped and phase inverted in aclass B, resistance coupled, pentode amplifier stage PS consisting of asingle pentode, with resistive anode load and with its grid biassed tothe anode cut-01f point. For each input pulse the amplifier stage PS isarranged to deliver negative-going pulses of about 50 volts amplitudethrough a condenser C1 to the cathode of a double diode V1. The cathodeof V1 is shown tied to the positive line voltage by a resistance R1 andthe anodes A1 and A2 are at the line voltage and well below the linevoltage respectively and vice versa as will hereinafter be explained.Thus a negative pulse applied to condenser Cr drives the cathode of V1negative and whichever of the anodes A1 or A2 which happens to be atline voltage will conduct. The arrangement is made more sensitive to 50volt pulses by the provision of a resistance Rla between the cathode ofV and earth such that the cathode is held at about 25 volts below thepositive line by the potential divider formed by R1 and Rm.

The double diode V1 and a double triode V211, V2b, together constitutean Eccles-Jordan scale-of-two circuit. This has two stable statesbeing asymmetrical circuitand is triggered from one to the other by theincoming pulses. For one stable state suppose that Va; is conducting, inwhich case the anode potential is low because of the potential dropacross R2. The grid potential of V2b is derived from the anode potentialof V23, by means of the resistive potential divider R4, R7, which is soproportioned that the grid potential of V21; is below cut-off. BecauseV2b is cut oflF, the grid bias of V241 is determined by the resistivepotential divider formed of R3, R5, Re, which is so proportioned that(having regard to the voltages of the positive and negative supplylines) the grid of V211 would be at about +20 v. with respect to the(common) earthed cathode, were it not for grid current which flows tosuch an extent that the grid is actually held at about cathode potential(because the grid-cathode resistance for grid positive is small comparedto the resistance of the potential divider).

The stable state just described would also apply were V2a substitutedfor V2b and vice versa.

The application of a negative pulse of sufficient ampli tude via V1causes the circuit to change from one of its stable states to the other,as is well known in the art.

Particular attention is directed to the potentials of the grids of V2aand V21), one of which is held at earth potential by grid current, theother being below cut off potential. On the arrival of a triggeringpulse, these grid potentials are interchanged. The grids of V2a, V2b areconnected directly to those of VBa, Vsb, a pair of triodes whose commoncathodes are connected to the negative supply line via resistance RB.Because the oathode potential of a triode connected in this way must bewithin a few volts (positive) of the grid voltage when the valve isconducting, and because the grid of the conducting member of the pair isheld at earth potential, as described above, the common cathodepotential must be a few volts above earth potential, and hence thecurrent in R is defined, both of its terminals being connected to stablevoltage points. All the cathode current flows through one only of V3a,V3b, the other being cut-off by the negative grid bias supplied from thegrid of the nonconducting member of the pair V28, V2b. The defined ofthe current in VQaVsbJ cathode current flows through one of the anodeloads,

R9, R10, the voltage drop across which is the product of the current andthe valueiof the resistance; if the latfter is fixed, then this voltagedrop is fixed. The anode voltage of the non-conducting member of V32,V3b is at the potential of the positive line, since no current flowsthrough the anode load.

On the arrival of a train of pulses at the input they are switched bythe valve V1 alternately to valve VZa or V211 the anode potentials ofwhich control the potentials of.

the diode anodes A1 and A2. Successive pulses cause the reversal of thestable states of VZaVZb and consequently, because of the conditions justdescribed, cause the currentto-be switched from V33, to V3b, and viceversa. In the interval between pulses, there is no change in the path 7Consequently, considering for example, the anode of Vsa, and supposingit to be 7 initially at the +HT potential (the valve being cut-off) onthe'arrival of a pulse the anode potential falls by a defined amount,and remains there until'the arrival of the next pulse and so on. Thewaveform at the anode of V33. consistsof a rectangular waveform ofdefined amplitude, but 'whose vertical sides (as seen on a cathode-rayoscilloscope) are in synchronism with the incoming pulses.

"For the purposes of the succeeding ratemeter part of 1 'the circuit,the waveform necessary for one cycle of its actionconsistsof a positivegoing edge followed, after 7 a' short interval',;by a negative goingedge. Such awave formcould be generated from each incoming pulse by atriggered square wave generator, having a pulse duration fixed bycircuit parameters and not by pulse repetition rateas in the'aboveexample. In such a case there is a chance that some pulses would gounrecorded because of their arrival during the generating time of thelected, and, the voltage change recorded on is the 7 to discharge acondenser Gig. in the brief'interval of time following anegative'movement of the'anodeof V32.-

The action of the double diode V4 is to extract a fixed charge perpulse: (actually per alternate pulse of the original train) from thecapacitor C52.- As explained above this results in an equilibriumcurrent in R16 (and. hence an equilibrium potential difference acrossR16): which bears a linear relation to the count rate. Theout:

put voltmeter is, therefore, deflected from'its Zero position an amountproportional to count rate. 7

As explained above the waveform at the" anode of V321 consists ofaseries of rectangular waves whose sides are coincident with theincoming'pulses, and whose amplitude is defined. On the positivemovement of the anode waveform of Vsa, the right hand plate of Ciaisheld at earth potential by conduction of the diode V411.

On the negative movement, first V ialS rendered non conducting becauseits anode is driven negative. After a 7 negative increment of about 2volts the diode 'vib is square wave (assuming random-in-time pulses).But

with the vcircuit'described this cannot happen, because eachpulsecausesbut one edge of a rectangular waveform. Only one half as manyrectangular waveforms are generated as there are input pulses, butthis'is corrected in the subsequent circuits by doubling the value ofone. of. the two components which are associated in the gen eration of avoltage proportional to the count rate.

The ratemeter part of the circuit comprises a double diode V4, aresistance-coupled pentode amplifier V5 of high gm'n (-l or more) 'an da triode Vs connected as a cathodefollower'. A potential dlVldCffRlZRlKconnected between the anode of V and the negative. supply line suppliesthe 'gr'id potential of the cathode follower Vs, 'whosecathode loadR14R15 is also returned to the negative supply line. From a tapping onthe cathode load (between R14R1 5) is connecteda parallel resistancecapacitance circuitRisOe'a, the other terminal of which isconnected tothe'gridof V5. 'By adjustment of the variable part (R14) of the'cathodeload of Va it is possible 7 to find a condition where the grid biasapplied to V5 is the appropriate value to adjust the anode voltage of V5to the -value required to adjust the grid voltage of V to the valuerequired to adjust the cathode voltage of V6 to earth potential. V thenreads zero, and R14 is seen i to have the function of a zero adjuster.In a practical case, the grid bias of V5 is about .2 or 3 volts,negative to the earthed cathode, and R14 is very small compared to R15;7 If now a source of voltage be introduced in series with'the networkconnecting the grid of V5 to the cathode load tapping of V6 (disturbingthe equilibrium of the circuit) the circuit of V5 V6 will settle down toa new state of equilibrium. The grid of V5 and the cathode of Va bothalter in'potential until the potential difierence between the grid of V5and the tapping point on the cathode load of Vsis equal to the voltageof the source introduced between these points, butbecause V constitutesan amplifier of considerable gain, the grid change is very much lessthan the cathode tap change, and if the 7 stage gain is sufiicientlyhigh, the grid change maybe negrendered conducting and current flowsinto Cia to'maintain its right hand plate at the grid potential of V5.Since the extent of this negative swing. is; defined, the total chargetransferred to V42. is also defined. Provided that C52. is much greaterthanC ia, little or no pulse waveform appears at the grid of V5.Furthermore, the potential change of the grid of V5 is always negligibleas explained above and constitutes in practice a fixed reference po- 7tential.

Considering the circuit of VsV starting from zero, onthe extraction ofcharges as described first a potential difference builds up on C53. Thisgoes'on increasing until the current in R1 results in a fall ofpotential across (15 at exactly the same rate as it is being built up;an

equilibrium state is then reached.

In mathematical terms, the action of the circuit, in

the equilibrium state, is given by where e is the reading of the outputvoltmeter E is the amplitude of the rectangular. Waves at the anode OfV35. t eg is the value of negative bias on the grid of V5 R16 is theresistance of the leak resistor R14R15 are theresistances inthecathodeload of Vs C45, is the. capacitance of the capacitor so 'rnarked; j

n is the count rate of theincoming pulses A typical set of practicalvalues'isz' ll-: v. (adjusted for scale-setting by variation of Rs) eranges from 0 to 50 volts on is determined with regard to the maximumvaluej' of n which it is desired to read on V In 'the absence of anyinput pulses,the uncertainties of the zero reading of V are about :25millivolts because of changes of valve operating pointswith agqheatervolt ages, etc. which is only i /z part in 1000 of the full scaledeflection. This is unreadable onan ordinary meter: The zero. stabilityis, therefore,.-good.

The function of 5a is to smooththe-pulsating'eurrents through the CSaRlScombination into a relatively smooth direct current in R16. It also hasthe effect of making the response of the instrument to changes of countrate exponential in character, with a time constant given by the productof CSaRlG. The circuit shows how another capacitor C5b may be connectedin parallel with C545, by moving a switch S to the right to increase theintegrating time. This is of particular use in smoothing out the randomfluctuating of the count rate of pulses derived from radioactivesources. In the left hand position in which the switch S2 is shown thecondenser C511 is not in use but is connected to the junction ofresistances R17 and R18, the potential of which is adjusted to be equalto that of the grid of V5 so that no change of potential occurs acrossthe measuring resistance R16 at the moment of connecting C512. Thestandby condenser has little effect on the integrating time when not inuse, as the output of V6 is of low impedance. A plurality of condensersC5 may be employed to give a plurality of integrating times, a suitablemultipoint switch being used to perform the function of S2. Similarly aplurality of condensers C; may be employed to give a plurality ofinstrument ranges. They may be connected to either R9 or R and arepreferably distributed between R9 and R10 to reduce stray capacityloading.

The component values in the circuits of V3, V4, V5, V6, are chosen sothat full scale deflection of the output meter occurs for between 50 and100 volts change at the cathode of V6, so that the inevitablecircuit-drifts due to ageing of valves, heater voltage changes due tounsteady mains voltages, component changes due to varying temperatures,and so on (which circuit-drifts usually amount to :50 millivolts) are ofminor importance in causing errors in the system.

Typical types and values for the components shown in the diagram are, byway of example only as follows:

Circuit component list V1 British CV 140.

V2 British CV 858 or 616.

V3 British CV 858 or 616.

V4 British CV 140.

V5 British CV 432 or 617.

V6 British CV 858 or 616 both halves in parallel.

Capacitors C1 .001 pf.

C2 39 pf.

(3 200 pf for full scale reading 1000 p. p.

second.

C 2000 pf. for full scale reading 100 p. p.

second.

C5aC5b 0.1 to 8 f. or more, depending on the desired smoothing of randomfluctuations of count rate and/ or the desired response time-constant ofthe output indications.

Resistors R1 27K. Rza 270K R2 47K R 47K. R4 500K.

Rs 330K.

Rs 25K.

R9 10 to 15K adjusted to set full scale deflection to a known pulserate.

R10 10 to 15K adjusted to set full scale deflection to a known pulserate.

R14 5K variable Zero adjustment.

R16 10MB.

R17 10K variable (set to 3 to 5K as required).

Voltrneter 50 v. full scale deflection 1 m. a.

I claim:

1. A pulse rate measuring circuit comprising, means deriving constantamplitude voltage steps at a rate related to the input pulse rate, meansapplying the voltage steps to a first plate of a feed condenser havingthe second plate connected to an electrode of one polarity of a firstdiode and to an electrode of opposite polarity of a second diode, areservoir condenser having a first plate connected to the otherelectrode of one of said diodes, a resistance in shunt with saidreservoir condenser to form a rate circuit, a high gain negative feedback D. C. amplifier having an input grid forming a substantially fixedreference point of potential, means connecting the first plate of saidreservoir condenser to said input grid, means connecting the remainingdiode electrode to a further substantially fixed reference point ofpotential having a low impedance to earth, means connecting the secondplate of said reservoir condenser to the output of said amplifier, andvoltage measuring means connected across the output of said amplifier.

2.. A pulse rate measuring circuit according to claim 1, wherein saidmeans for deriving voltage steps constant in amplitude comprises aconstant amplitude square wave generating circuit having two stablestates, being changed from one stable state to the other by successiveinput pulses.

3. A pulse rate measuring circuit according to claim 1, having a secondreservoir condenser, switch means for connecting said second condenserin parallel with the first mentioned reservoir condenser, and switchmeans associated with a fixed potential point for maintaining saidsecond condenser charged to the potential of first mentioned condenserwhen not connected in parallel therewith.

References Cited in the file of this patent UNITED STATES PATENTS2,113,011 White Apr. 5, 1938 2,307,316 Wolff Ian. 5, 1943 2,403,557Sanders July 9, 1946 2,487,191 Smith Nov. 8, 1949 2,513,668 Parker July4, 1950 2,540,524 Houghton Feb. 6, 1951 2,573,150 Lacy Oct. 30, 19512,580,083 Doba, Jr. et al. Dec. 25, 1951

